Paleoecology and Taphonomy of Full-Glacial Terrestrial Gastropods from the Latch Valley, West-Central Wisconsin

The use of gastropods in reconstructing the late Pleistocene environment of Wisconsin is hampered by two factors. The first is a lack of fossil sites and the second is that the taphonomic biases operating on terrestrial gastropod assemblages are not well understood. This project is a study of the characteristics of shell assemblages collected from two sites (Kulas Quarry and Hwy-JJ) within the Latch Valley, a tributary valley of the Trempealeau River in west-central Wisconsin. Kulas Quarry, is a 7.5 m by 21 m wide exposure of alluvial valley fill dominated by tabular and trough cross-bedded sand units. Hwy-JJ is a 1.5 m tall by 4 m wide exposure of colluvial silt. Species richness reached a maximum of ten terrestrial gastropod species at Kulas Quarry, but only seven species at Hwy-JJ. The succineid gastropod Catinella gelida dominated the assemblages at both sites. However, the abundances of several other taxa were different between the two sites. Gastropods favoring cold, moist habitats such as Columella columella alticola, Vertigo modesta modesta, and Discus shimeki are found in much greater proportion within alluvial sediments. Pupilla muscorum, which favors cold, open and more xeric conditions occurs in much greater proportions within colluvial samples. The relict Hendersonia occulta occurred in approximately half of all alluvial samples, but was absent from samples of colluvium. The proportion of broken individuals corresponds directly with shell morphology. Large taxa and minute Vertigo shells occur predominantly as intact shells, while the thin-shelled columellar shells were the most likely to occur as broken shells. Interestingly, the large-shelled H. occulta only occurred as broken fragments. Species frequency appears to be linked to environmental factors, such as moisture, while shell breakage appears strongly linked to shell morphology except for H. occulta whose preferential destruction suggests a separate taphonomic influence.